JP5928669B1 - Ferritic stainless steel and manufacturing method thereof - Google Patents

Abstract

Ferrite crystal grains satisfying any one or both of C component: 2CC or more and N concentration: 2CN or more are set to 5% to 50% by volume ratio with respect to the entire structure, and Vickers hardness is set. By setting it to 180 or less, a ferritic stainless steel that is excellent in formability and ridging resistance and can be manufactured with high productivity is provided. Here, CC and CN are the contents (mass%) of C and N in the steel, respectively.

Description

  The present invention relates to a ferritic stainless steel excellent in formability and ridging resistance.

  Since ferritic stainless steel represented by SUS430 is economical and excellent in corrosion resistance, it is used in home appliances, kitchen appliances, and the like. In recent years, since there is magnetism, the application to the cooking utensil which can respond to an IH (induction heating) system is increasing. Cooking utensils such as pots are often formed by overhanging, and sufficient elongation is required to form a predetermined shape.

  On the other hand, the appearance of the surface of cooking pots greatly affects the product value. Usually, when ferritic stainless steel is molded, surface irregularities called ridging are formed, and the surface appearance after molding deteriorates. When excessive ridging is generated, a polishing step for removing irregularities after molding is required, which increases the manufacturing cost. Therefore, it is required that the ridging is small. Ridging is caused by an aggregate of ferrite grains having a similar crystal orientation (hereinafter sometimes referred to as a ferrite colony or a colony). The coarse columnar crystal structure produced during casting is expanded by hot rolling, and the expanded grains or grain groups remain even after hot-rolled sheet annealing, cold-rolling and cold-rolled sheet annealing. It is thought to be formed.

  For example, in Patent Literature 1, “mass%, C: 0.02 to 0.12%, N: 0.02 to 0.12%, Cr: 16 to 18%, V: 0.01 to 0.15%, and Al: 0.03%, for example, The steel material containing the following is heated, hot rolling is performed so that the rolling end temperature FDT is in the range of 1050 to 750 ° C., cooling is started within 2 seconds after the hot rolling ends, and the cooling rate is 10 to 150 ° C. / After cooling to 550 ° C. or lower after s, the steel sheet is rolled up, made into a ferrite + martensite structure, or further subjected to a pre-rolling process in which cold or warm rolling is performed at a reduction ratio of 2 to 15%, "A method for producing ferritic stainless steel" is disclosed. Here, it is said that instead of rapid cooling after hot rolling, rapid cooling after winding may be performed to obtain a ferrite + martensite structure.

  In Patent Document 2, “mass%, C: 0.01 to 0.08%, Si: 0.30% or less, Mn: 0.30 to 1.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.02% or less, N: 0.01 to 0.08%, Cr: 16.0 to 18.0%, the balance is composed of Fe and inevitable impurities, and the structure is composed of ferrite crystal grains on which Cr carbonitride is precipitated, and the rolling direction In the cross section formed by the plate thickness direction, the ratio Dz / Dl of the average ferrite crystal grain size Dz in the plate thickness direction and the average ferrite crystal grain size Dl in the rolling direction is 0.7 or more and occupies the observation field of Cr carbonitride A ferritic stainless steel cold-rolled steel sheet having an area ratio Sp of 2% or more and an average equivalent circle diameter Dp of 0.5 μm or more is disclosed. In addition, Sp and Dp of Cr carbonitride were obtained by observing 2000 times with SEM.

JP 2001-98328 A JP 2009-275268 JP

  However, in the method of Patent Document 1, since it is necessary to perform preliminary rolling before hot-rolled sheet annealing when manufacturing a steel sheet, there remains a problem in that the rolling load increases and productivity decreases.

  In addition, the steel sheet described in Patent Document 2 has an average equivalent circle radius of Cr carbonitride deposited on the finish-annealed plate, which is as coarse as 0.5 μm or more. There was a risk of defects.

  The present invention has been developed in view of the above-described present situation, and provides ferritic stainless steel that is excellent in formability and ridging resistance and can be manufactured under high productivity, together with its manufacturing method. The purpose is to do.

  The “excellent formability” means that the elongation at break (El) in a tensile test based on JIS Z 2241 is perpendicular to the rolling direction (hereinafter sometimes referred to as rolling perpendicular direction) as the longitudinal direction. And 25% or more, preferably 28% or more, more preferably 30% or more.

  In addition, “excellent ridging resistance” means that the ridging height measured by the method described below is 2.5 μm or less. For measuring the ridging height, first, a JIS No. 5 tensile test piece is taken in parallel with the rolling direction. Next, after polishing the surface of the collected test piece using # 600 emery paper, 20% tensile strain is applied. Next, the arithmetic mean waviness Wa defined by JIS B 0601 (2001) is measured with a surface roughness meter in the direction perpendicular to the rolling direction on the polishing surface at the center of the parallel part of the test piece. The measurement conditions are a measurement length of 16 mm, a high cut filter wavelength of 0.8 mm, and a low cut filter wavelength of 8 mm. This arithmetic mean swell is defined as the ridging height.

In order to solve the above-mentioned problems, the inventors have made extensive studies. In particular, in order to increase productivity, the inventors have not used long-time hot-rolled sheet annealing by box annealing (batch annealing), which is generally performed at present, but short-time hot-rolled sheets using a continuous annealing furnace. Intensive study was conducted on a method for ensuring excellent moldability and ridging resistance by annealing.
As a result, even when performing hot-rolled sheet annealing for a short time using a continuous annealing furnace, by generating a predetermined amount of martensite phase during hot-rolled sheet annealing, by performing cold rolling in that state, It has been found that ferrite colonies formed at the casting stage can be effectively destroyed.
Furthermore, the cold-rolled sheet thus obtained is subjected to cold-rolled sheet annealing in the ferrite single-phase temperature range, so that at least one of C and N starting from the martensite phase generated during hot-rolled sheet annealing. Ferrite crystals with a low carbonitride concentration, starting from the ferrite crystal grains that are enriched in steel (hereinafter sometimes referred to as C / N-concentrated grains) and the portion that was the ferrite phase during hot-rolled sheet annealing It was found that a composite structure of grains (hereinafter sometimes simply referred to as non-concentrated grains) was obtained, and thereby excellent ridging resistance and moldability were obtained at the same time. Here, as a criterion for determining that at least one of C and N is concentrated in the ferrite crystal grains, at least one of the C and N concentrations in the ferrite crystal grains is C or N. It was found that the content in steel (mass%) is appropriate to be twice or more.
In other words, since a large amount of fine carbonitride precipitates in the C · N-concentrated grains during cold-rolled sheet annealing, grain growth during annealing is suppressed by the pinning effect, thereby preventing the accumulation of ferrite colonies. And ridging resistance is improved. On the other hand, in the non-concentrated grains, the C / N concentration is reduced, so that grain growth is promoted and elongation, that is, formability is improved.
The present invention was completed after further studies based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. In mass%, C: 0.005 to 0.030%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.0%, P: 0.040% or less, S: 0.010 %: Cr: 15.5 to 18.0%, Ni: 0.01 to 1.0%, Al: 0.001 to 0.05% and N: 0.005 to 0.06%, The balance consists of the component composition of Fe and inevitable impurities,
Further, the structure is composed of a structure other than the ferrite phase and the ferrite phase having a total volume ratio of less than 1% with respect to the entire structure.
In the ferrite phase, the ferrite crystal grains satisfying one or both of C concentration: 2C _C or more and N concentration: 2C _N or more are 5% or more and 50% or less by volume ratio with respect to the entire structure,
Ferritic stainless steel having a Vickers hardness of 180 or less and a ridging height of 2.5 μm or less.
Here, C _C and C _N are the contents (mass%) of C and N in the steel, respectively.

2. The component composition is further one percent by mass selected from Cu: 0.01 to 1.0%, Mo: 0.01 to 0.5%, and Co: 0.01 to 0.5%. 2. The ferritic stainless steel according to 1 above, containing two or more kinds.

3. The component composition is further in mass%, V: 0.01 to 0.25%, Ti: 0.001 to 0.10%, Nb: 0.001 to 0.10%, Ca: 0.0002 to 0 0020%, Mg: 0.0002 to 0.0050%, B: 0.0002 to 0.0050%, and REM: 0.01 to 0.10% selected from one or more The ferritic stainless steel according to 1 or 2 above.

4). In the component composition, the C content is 0.005 to 0.030 mass%, the Si content is 0.25 mass% to less than 0.40 mass%, and the Mn content is 0.05 to 0.35 mass%. %
The volume fraction of the ferrite crystal grains is 5% or more and 30% or less,
The ferritic stainless steel according to any one of 1 to 3 above, wherein the elongation at break in a direction perpendicular to the rolling direction is 28% or more and the ridging height is 2.5 μm or less.

5. In the component composition, the C content is 0.005 to 0.025 mass%, the Si content is 0.05 mass% or more and less than 0.25 mass%, and the Mn content is 0.60 to 0.90 mass. %, N content is 0.005 to 0.025 mass%,
The volume fraction of the ferrite crystal grains is 5% or more and 20% or less,
The ferritic stainless steel according to any one of 1 to 3 above, wherein the elongation at break in a direction perpendicular to the rolling direction is 30% or more and the ridging height is 2.5 μm or less.

6). A method for producing the ferritic stainless steel according to any one of 1 to 5,
Hot-rolling a steel slab comprising the component composition according to any one of 1 to 5 to form a hot-rolled sheet; and
Performing the hot-rolled sheet annealing by holding the hot-rolled sheet in a temperature range of 920 ° C. or higher and 1050 ° C. or lower for 5 seconds to 15 minutes to obtain a hot-rolled annealed sheet;
Cold rolling the hot-rolled annealed plate to form a cold-rolled plate;
A method for producing ferritic stainless steel, comprising a step of performing cold-rolled sheet annealing in which the cold-rolled sheet is held in a temperature range of 800 ° C. or higher and lower than 900 ° C. for 5 seconds to 5 minutes.

7). In the component composition, the C content is 0.005 to 0.030 mass%, the Si content is 0.25 mass% to less than 0.40 mass%, and the Mn content is 0.05 to 0.35 mass%. %
The holding temperature in the hot-rolled sheet annealing is 940 ° C. or higher and 1000 ° C. or lower,
The manufacturing method of the ferritic stainless steel of said 6 whose holding temperature in the said cold rolled sheet annealing is 820 degreeC or more and less than 880 degreeC.

8). In the component composition, the C content is 0.005 to 0.025 mass%, the Si content is 0.05 mass% or more and less than 0.25 mass%, and the Mn content is 0.60 to 0.90 mass. %, N content is 0.005 to 0.025 mass%,
The holding temperature in the hot-rolled sheet annealing is 960 ° C. or higher and 1050 ° C. or lower,
The manufacturing method of the ferritic stainless steel of said 6 whose holding temperature in the said cold rolled sheet annealing is 820 degreeC or more and less than 880 degreeC.

According to the present invention, ferritic stainless steel having excellent formability and ridging resistance can be obtained.
In addition, the ferritic stainless steel of the present invention can be manufactured not by long-time hot-rolled sheet annealing by box annealing (batch annealing) but by short-time hot-rolled sheet annealing using a continuous annealing furnace, This is extremely advantageous in terms of productivity.

Hereinafter, the present invention will be specifically described.
First, the reason why the ferritic stainless steel of the present invention has excellent formability and ridging resistance will be described.
In order to improve the ridging resistance properties of stainless steel, it is effective to destroy ferrite colonies that are aggregates of crystal grains having similar crystal orientations.
From the viewpoint of productivity, the present inventors do not perform long-time hot-rolled sheet annealing by box annealing (batch annealing) which is generally performed at present, but short-time hot-rolled sheet annealing using a continuous annealing furnace. As a result of repeated investigations to ensure excellent formability and ridging resistance, the temperature of the ferrite phase and austenite phase was raised to the two-phase temperature range during hot-rolled sheet annealing to promote recrystallization and generate an austenite phase. By securing a predetermined amount of martensite phase after hot-rolled sheet annealing and cold rolling a hot-rolled annealed sheet containing this predetermined amount of martensite phase, rolling distortion is effectively imparted to the ferrite phase, It has been found that ferrite colonies are efficiently destroyed.

  Furthermore, the present inventors appropriately control the component composition, hot-rolled sheet annealing conditions and cold-rolled sheet annealing conditions, and make the cold-rolled sheet annealed structure a composite structure of C · N concentrated grains and non-concentrated grains. Thus, it has been found that it is possible to further improve the ridging resistance and obtain sufficient moldability. The C / N concentrated grains are ferrite grains in which martensite generated during hot rolling annealing is decomposed. When heated to a two-phase region (ferrite + austenite) during hot-rolled sheet annealing, C and N are concentrated into an austenite phase having a larger solid solubility limit than the ferrite phase. Thereafter, when cooled, the austenite phase is transformed into a martensite phase in which C and N are concentrated. Such a hot-rolled annealed sheet containing a martensite phase is annealed in the ferrite single phase temperature range after cold rolling, and the martensite phase is decomposed to obtain C / N concentrated grains. Since a large amount of carbonitride precipitates on the C · N-concentrated grains, grain growth is hindered by a pinning effect during cold rolling. Thereby, it is considered that excessive texture accumulation of ferrite grains is prevented, and ridging resistance is greatly improved. This effect is obtained when at least one of C and N is concentrated to at least twice the content (mass%) in the steel. On the other hand, ferrite grains (non-concentrated grains) other than C / N-concentrated grains have a C and N concentration lower than the steel content (mass%). Will improve. This makes it possible to achieve both excellent ridging resistance and sufficient moldability.

However, when the volume ratio of the C / N concentrated grains increases beyond a certain level, the strength increases excessively and the breaking elongation decreases. Therefore, the inventors conducted a detailed study on the volume ratio of C / N-concentrated grains that can provide excellent moldability and ridging resistance.
As a result, by controlling the volume ratio of C / N concentrated grains after cold-rolled sheet annealing to a range of 5 to 50% in terms of the volume ratio with respect to the entire structure, without accompanied by a decrease in breaking elongation due to an increase in steel sheet strength. The inventors have found that predetermined moldability and ridging resistance can be obtained. In particular, when considering the balance between formability and ridging resistance, the volume ratio of the C · N concentrated grains is preferably 5 to 30% in terms of the volume ratio with respect to the entire structure. Further, from the viewpoint of obtaining better moldability, the volume ratio of the C / N concentrated grains is preferably 5 to 20% in terms of the volume ratio with respect to the entire structure. It should be noted that the structure other than ferrite grains composed of C / N concentrated grains basically becomes ferrite grains composed of non-concentrated grains, but other structures (such as martensite phase) have a volume ratio of the entire structure. A total of less than 1% is acceptable.

In addition, if the holding temperature and holding time of cold-rolled sheet annealing are insufficient, not only recrystallization of ferrite grains will be insufficient, but also the decomposition of the martensite phase generated during hot-rolled sheet annealing will be insufficient, and elongation will occur. Decreases. In order to obtain sufficient formability, it is necessary to sufficiently complete recrystallization after the cold-rolled sheet annealing and sufficiently decompose the martensite phase generated during the hot-rolled sheet annealing. On the other hand, when the holding temperature in cold-rolled sheet annealing is too high, a new martensite phase is generated and elongation is lowered. Therefore, it is necessary to suppress the abundance of the martensite phase. The martensite phase must be less than 1% by volume with respect to the entire structure. In order to obtain excellent moldability, 0% is preferable.
As a result of the study by the present inventors, it has been found that in order to avoid these problems and obtain an appropriate structure, the cold-rolled sheet annealing conditions should be appropriately controlled and the Vickers hardness should be 180 or less. . Preferably, the Vickers hardness is 165 or less.

Next, the reason for limiting the component composition in the ferritic stainless steel of the present invention will be described. The unit of element content in the component composition is “mass%”, but hereinafter, it is simply indicated by “%” unless otherwise specified.
C: 0.005 to 0.050%
C is an important element for generating C · N concentrated grains and improving ridging resistance. It also has the effect of promoting the formation of the austenite phase and expanding the two-phase temperature range of the ferrite phase and the austenite phase during hot-rolled sheet annealing. In order to obtain these effects, 0.005% or more of C is necessary. However, if the C content exceeds 0.050%, the steel sheet becomes hard and a predetermined elongation at break cannot be obtained. Therefore, the C content is in the range of 0.005 to 0.050%. Further, from the viewpoint of further improving elongation at break and obtaining excellent moldability, the C content is 0.005 to 0.030% or 0.005 to 0 depending on the Si content and Mn content described later. A range of 0.025% is preferable. More preferably, it is 0.008 to 0.025% of range, still more preferably 0.010 to 0.020% of range.

Si: 0.01-1.00%
Si is an element that acts as a deoxidizer during steel melting. In order to obtain this effect, addition of 0.01% or more of Si is necessary. However, if the Si content exceeds 1.00%, the steel plate becomes hard and a predetermined elongation at break cannot be obtained. Furthermore, since the surface scale produced | generated at the time of annealing becomes strong and pickling becomes difficult, it is not preferable. Therefore, the Si content is in the range of 0.01 to 1.00%. Preferably it is 0.05 to 0.75% of range. More preferably, it is 0.05 to 0.40% of range.
In addition, when Mn content described later is in the range of 0.05 to 0.35%, from the viewpoint of obtaining excellent formability by further improving the elongation at break while ensuring the predetermined ridging resistance characteristics, The amount is preferably in the range of 0.25% or more and less than 0.40%.
In addition, when the Mn content to be described later is in the range of 0.60 to 0.90%, from the viewpoint of obtaining excellent formability by further improving the elongation at break while ensuring the predetermined ridging resistance characteristics, The amount is preferably 0.05% or more and less than 0.25%.

Mn: 0.01 to 1.0%
Mn promotes the formation of an austenite phase like C, and has the effect of expanding the two-phase temperature range of the ferrite phase and the austenite phase during hot-rolled sheet annealing. In order to obtain this effect, it is necessary to add 0.01% or more of Mn. However, if the Mn content exceeds 1.0%, the amount of MnS produced increases and the corrosion resistance decreases. Therefore, the Mn content is in the range of 0.01 to 1.0%. Preferably it is 0.05 to 0.90% of range.
As described above, when the Si content is in the range of 0.25% or more and less than 0.40%, the moldability is further improved by improving the elongation at break while ensuring the predetermined ridging resistance. From the viewpoint, the Mn content is preferably in the range of 0.05 to 0.35%.
In addition, when the Si content is in the range of 0.05% or more and less than 0.25%, Mn is contained from the viewpoint of improving the elongation at break and obtaining excellent formability while ensuring the predetermined ridging resistance characteristics. The amount is preferably in the range of 0.60 to 0.90%. More preferably, it is 0.70 to 0.90% of range. More preferably, it is 0.75 to 0.85% of range.

P: 0.040% or less Since P is an element that promotes grain boundary fracture due to grain boundary segregation, the lower one is desirable, and the upper limit is made 0.040%. Preferably it is 0.030% or less. More preferably, it is 0.020% or less. In addition, although the minimum of P content is not specifically limited, From viewpoints, such as manufacturing cost, it is about 0.010%.

S: 0.010% or less S is an element that exists as sulfide inclusions such as MnS and decreases ductility, corrosion resistance, etc., and particularly when the content exceeds 0.010%, their adverse effects Is noticeable. For this reason, the S content is desirably as low as possible, and the upper limit of the S content is 0.010%. Preferably it is 0.007% or less. More preferably, it is 0.005% or less. In addition, although the minimum of S content is not specifically limited, From viewpoints, such as manufacturing cost, it is about 0.001%.

Cr: 15.5 to 18.0%
Cr is an element having an effect of improving the corrosion resistance by forming a passive film on the surface of the steel sheet. In order to acquire this effect, it is necessary to make Cr content 15.5% or more. However, if the Cr content exceeds 18.0%, the austenite phase is not sufficiently generated during hot-rolled sheet annealing, and predetermined material characteristics cannot be obtained. Therefore, the Cr content is in the range of 15.5 to 18.0%. Preferably it is 16.0 to 17.5% of range. More preferably, it is 16.5 to 17.0% of range.

Ni: 0.01 to 1.0%
Ni, like C and Mn, promotes the formation of an austenite phase and has the effect of expanding the two-phase temperature range in which a ferrite phase and an austenite phase appear during hot-rolled sheet annealing. In order to obtain this effect, the Ni content needs to be 0.01% or more. However, if the Ni content exceeds 1.0%, the workability decreases. Therefore, the Ni content is in the range of 0.01 to 1.0%. Preferably it is 0.1 to 0.6% of range. More preferably, it is 0.1 to 0.4% of range.

Al: 0.001 to 0.10%
Al is an element that acts as a deoxidizing agent similarly to Si. In order to obtain this effect, the Al content needs to be 0.001% or more. However, when the Al content exceeds 0.10%, Al-based inclusions such as Al ₂ O ₃ increase, and the surface properties tend to decrease. Therefore, the Al content is in the range of 0.001 to 0.10%. Preferably it is 0.001 to 0.05% of range. More preferably, it is 0.001 to 0.03% of range.

N: 0.005 to 0.06%
N is an important element for generating C.N concentrated grains and improving ridging resistance. It also has the effect of promoting the formation of the austenite phase and expanding the two-phase temperature range in which the ferrite phase and austenite phase appear during hot-rolled sheet annealing. In order to acquire this effect, it is necessary to make N content 0.005% or more. However, when the N content exceeds 0.06%, the ductility is remarkably lowered and the corrosion resistance is reduced by promoting the precipitation of Cr nitride. Therefore, N content is taken as 0.005 to 0.06% of range. Preferably it is 0.005 to 0.05% of range. More preferably, it is 0.005 to 0.025% of range. More preferably, it is 0.010 to 0.025% of range. Still more preferably, it is 0.010 to 0.020% of range.
In particular, when the C content is 0.005 to 0.025%, the Si content is 0.05% or more and less than 0.25%, and the Mn content is in the range of 0.60 to 0.90%, The N content is preferably in the range of 0.005 to 0.025%. More preferably, it is 0.010 to 0.025% of range. More preferably, it is 0.010 to 0.020% of range.

  Although the basic components have been described above, the ferritic stainless steel of the present invention can appropriately contain the elements described below as needed for the purpose of improving manufacturability or material properties.

One or more selected from Cu: 0.01 to 1.0%, Mo: 0.01 to 0.5%, and Co: 0.01 to 0.5% Cu: 0.01 to 1 0.0%, Mo: 0.01 to 0.5%
Cu and Mo are both elements that improve the corrosion resistance, and it is effective to contain them particularly when high corrosion resistance is required. In addition, Cu has an effect of promoting the generation of an austenite phase and expanding a two-phase temperature range in which a ferrite phase and an austenite phase appear during hot-rolled sheet annealing. Each of these effects can be obtained with a content of 0.01% or more. However, if the Cu content exceeds 1.0%, the hot workability may decrease, which is not preferable. Therefore, when it contains Cu, it is set as 0.01 to 1.0% of range. Preferably it is 0.2 to 0.8% of range. More preferably, it is 0.3 to 0.5% of range. On the other hand, when the Mo content exceeds 0.5%, the austenite phase is not sufficiently generated during annealing, and predetermined material characteristics cannot be obtained, which is not preferable. Therefore, when it contains Mo, it is set as 0.01 to 0.5% of range. Preferably it is 0.2 to 0.3% of range.

Co: 0.01 to 0.5%
Co is an element that improves toughness. This effect is obtained by adding 0.01% or more of Co. On the other hand, if the Co content exceeds 0.5%, the productivity is lowered. Therefore, when it contains Co, it is set as 0.01 to 0.5% of range. More preferably, it is 0.02 to 0.20% of range.

V: 0.01 to 0.25%, Ti: 0.001 to 0.10%, Nb: 0.001 to 0.10%, Ca: 0.0002 to 0.0020%, Mg: 0.0002 to One or more selected from 0.0050%, B: 0.0002 to 0.0050% and REM: 0.01 to 0.10% V: 0.01 to 0.25%
V combines with C and N in the steel to reduce solute C and N. Thereby, the precipitation of carbonitrides on the hot-rolled sheet is suppressed to suppress the occurrence of linear flaws due to hot-rolling and annealing, and the surface properties are improved. In order to obtain these effects, the V content needs to be 0.01% or more. However, when the V content exceeds 0.25%, the workability is lowered and the manufacturing cost is increased. Therefore, when it contains V, it is set as 0.01 to 0.25% of range. Preferably it is 0.03 to 0.15% of range. More preferably, it is 0.03 to 0.05% of range.

Ti: 0.001-0.10%, Nb: 0.001-0.10%
Ti and Nb are elements having a high affinity with C and N, like V, and precipitate as carbides or nitrides during hot rolling, reducing the solid solution C and N in the matrix, There is an effect of improving workability after annealing. In order to obtain these effects, it is necessary to contain 0.001% or more of Ti or 0.001% or more of Nb. However, if the Ti content or Nb content exceeds 0.10%, good surface properties cannot be obtained due to the precipitation of excess TiN and NbC. Therefore, when Ti is contained, the range is 0.001 to 0.10%, and when Nb is contained, the range is 0.001 to 0.10%. The Ti content is preferably in the range of 0.003 to 0.010%. The Nb content is preferably in the range of 0.005 to 0.020%. More preferably, it is 0.010 to 0.015% of range.

Ca: 0.0002 to 0.0020%
Ca is an effective component for preventing nozzle clogging due to crystallization of Ti-based inclusions that are likely to occur during continuous casting. In order to acquire the effect, 0.0002% or more needs to be contained. However, if the Ca content exceeds 0.0020%, CaS is generated and the corrosion resistance decreases. Therefore, when it contains Ca, it is set as 0.0002 to 0.0020% of range. Preferably it is the range of 0.0005-0.0015. More preferably, it is 0.0005 to 0.0010% of range.

Mg: 0.0002 to 0.0050%
Mg is an element that has an effect of improving hot workability. In order to acquire this effect, 0.0002% or more needs to be contained. However, when the Mg content exceeds 0.0050%, the surface quality deteriorates. Therefore, when it contains Mg, it is set as 0.0002 to 0.0050% of range. Preferably it is 0.0005 to 0.0035% of range. More preferably, it is 0.0005 to 0.0020% of range.

B: 0.0002 to 0.0050%
B is an element effective for preventing embrittlement at low temperature secondary work. In order to acquire this effect, 0.0002% or more needs to be contained. However, when the B content exceeds 0.0050%, the hot workability decreases. Therefore, when it contains B, it is set as 0.0002 to 0.0050% of range. Preferably it is 0.0005 to 0.0035% of range. More preferably, it is 0.0005 to 0.0020% of range.

REM: 0.01-0.10%
REM (Rare Earth Metals) is an element that improves oxidation resistance, and in particular, has an effect of suppressing the formation of an oxide film at the weld and improving the corrosion resistance of the weld. In order to obtain this effect, it is necessary to add 0.01% or more of REM. However, when the REM content exceeds 0.10%, productivity such as pickling properties during cold rolling annealing is reduced. Moreover, since REM is an expensive element, excessive addition causes an increase in manufacturing cost, which is not preferable. Therefore, when it contains REM, it is set as 0.01 to 0.10% of range.

The component composition in the ferritic stainless steel of the present invention has been described above.
In addition, components other than the above among the component composition in the present invention are Fe and inevitable impurities.

Next, the manufacturing method of the ferritic stainless steel of this invention is demonstrated.
Molten steel having the above component composition is melted by a known method such as a converter, an electric furnace, a vacuum melting furnace or the like, and is made into a steel material (slab) by a continuous casting method or an ingot-bundling method. This slab is heated at 1100 to 1250 ° C. for 1 to 24 hours, or directly hot-rolled as cast without heating to obtain a hot-rolled sheet.
Then, hot-rolled sheet annealing is performed by holding the hot-rolled sheet for 5 seconds to 15 minutes at a temperature of 900 ° C. or higher and 1050 ° C. or lower, which is a two-phase region temperature of a ferrite phase and an austenite phase.
In the case of a component composition in which C: 0.005 to 0.030%, Si: 0.25% or more and less than 0.40%, and Mn: 0.05 to 0.35% (hereinafter simply referred to as Component Composition 1) It is also preferable to perform hot-rolled sheet annealing that is held at a temperature of 940 ° C. or higher and 1000 ° C. for 5 seconds to 15 minutes.
Further, C: 0.005 to 0.025%, Si: 0.05% or more and less than 0.25%, Mn: 0.60 to 0.90%, and N: 0.005 to 0.025% In the case of the composition (hereinafter, also simply referred to as component composition 2), it is preferable to perform hot-rolled sheet annealing that is held at a temperature of 960 ° C. or higher and 1050 ° C. or lower for 5 seconds to 15 minutes.

Next, the hot-rolled annealed sheet is pickled as necessary, and then cold-rolled to obtain a cold-rolled sheet. Then, cold-rolled sheet annealing is performed on the cold-rolled sheet to obtain a cold-rolled annealed sheet. Furthermore, it pickles as needed with respect to a cold-rolled annealing board, and is set as a product.
Cold rolling is preferably performed at a rolling reduction of 50% or more from the viewpoints of extensibility, bendability, press formability, and shape correction. In the present invention, cold rolling and annealing may be repeated twice or more. Moreover, cold-rolled sheet annealing is performed by holding at a temperature of 800 ° C. or higher and lower than 900 ° C. for 5 seconds to 5 minutes. In the case of the above component composition 1 or 2, it is preferable to hold at a temperature of 820 ° C. or higher and lower than 880 ° C. for 5 seconds to 5 minutes. Further, BA annealing (bright annealing) may be performed in order to obtain more gloss.
In order to further improve the surface properties, grinding or polishing may be performed.
Hereinafter, the reason for limitation of hot-rolled sheet annealing and cold-rolled sheet annealing conditions will be described.

Hot-rolled sheet annealing conditions: held at a temperature of 900 ° C. or higher and 1050 ° C. or lower for 5 seconds to 15 minutes Hot-rolled sheet annealing is an extremely important step for obtaining excellent moldability and ridging resistance characteristics of the present invention. When the holding temperature in hot-rolled sheet annealing is less than 900 ° C., sufficient recrystallization does not occur and the ferrite single-phase region is obtained, so that the effects of the present invention that are manifested by two-phase region annealing may not be obtained. On the other hand, when the holding temperature exceeds 1050 ° C., the volume ratio of the martensite phase generated after the hot-rolled sheet annealing decreases, so the concentration effect of rolling strain on the ferrite phase in the subsequent cold rolling is reduced, and the ferrite The destruction of the colony becomes insufficient, and the predetermined ridging resistance characteristic may not be obtained.
In addition, when the holding time is less than 5 seconds, even if annealing is performed at a predetermined temperature, generation of austenite phase and recrystallization of the ferrite phase do not occur sufficiently, so that desired formability may not be obtained. On the other hand, if the holding time exceeds 15 minutes, C enrichment in the austenite phase is promoted, and a martensite phase is excessively generated after hot-rolled sheet annealing, which may reduce hot-rolled sheet toughness. Therefore, hot-rolled sheet annealing is held at a temperature of 900 ° C. or higher and 1050 ° C. or lower for 5 seconds to 15 minutes. Preferably, it is held at a temperature of 920 ° C. or higher and 1000 ° C. or lower for 5 seconds to 15 minutes.
In addition, in the case of the above-mentioned component composition 1, it is more preferable to hold | maintain at the temperature of 940 degreeC or more and 1000 degrees C or less for 5 second-15 minutes. Moreover, in the case of the above component composition 2, it is more preferable to hold | maintain at the temperature of 960 degreeC or more and 1050 degrees C or less for 5 second-15 minutes. The upper limit of the holding time is more preferably 5 minutes, and further preferably 3 minutes.

Cold-rolled sheet annealing conditions: Hold for 5 seconds to 5 minutes at a temperature of 800 ° C. or higher and lower than 900 ° C. Cold-rolled sheet annealing recrystallizes the ferrite phase formed by hot-rolled sheet annealing and the volume ratio of C / N concentrated grains. This is an important process for adjusting to a predetermined range. If the holding temperature in cold-rolled sheet annealing is less than 800 ° C., recrystallization does not occur sufficiently and a predetermined breaking elongation cannot be obtained. On the other hand, when the holding temperature in cold-rolled sheet annealing is 900 ° C. or higher, a martensite phase is generated, the steel sheet is hardened, and a predetermined breaking elongation cannot be obtained.
In addition, when the holding time is less than 5 seconds, even if annealing is performed at a predetermined temperature, the ferrite phase is not sufficiently recrystallized, so that a predetermined elongation at break cannot be obtained. On the other hand, if the holding time exceeds 5 minutes, the crystal grains become extremely coarse and the glossiness of the steel sheet is lowered, which is not preferable from the viewpoint of surface beauty. Therefore, cold-rolled sheet annealing is held at a temperature of 800 ° C. or higher and lower than 900 ° C. for 5 seconds to 5 minutes. Preferably, it is held at 820 ° C. or more and less than 900 ° C. for 5 seconds to 5 minutes. In the case of the above component composition 1 or 2, it is preferable to hold at a temperature of 820 ° C. or higher and lower than 880 ° C. for 5 seconds to 5 minutes.

Steel having the component composition shown in Table 1 was melted in a 50 kg small vacuum melting furnace. These steel ingots were heated at 1150 ° C. for 1 h, and then hot-rolled to obtain 3.0 mm thick hot rolled sheets. After hot rolling, it was water cooled to 600 ° C. and then air cooled. Subsequently, these hot-rolled sheets were subjected to hot-rolled sheet annealing under the conditions shown in Table 2, and then the surfaces were descaled by shot blasting and pickling. Furthermore, after cold-rolling to a plate thickness of 0.8 mm, cold-rolled sheet annealing was performed under the conditions described in Table 2, and descaling treatment was performed by pickling to obtain a cold-rolled annealed sheet.
The following evaluation was performed on the cold-rolled annealed sheet thus obtained.

(1) Volume ratio of C / N concentrated grains The volume ratio of C / N concentrated grains was measured using EPMA (Electron Beam Microanalyzer [JEOL JXA-8200]). A test piece having a width of 10 mm and a length of 15 mm was cut out from the central portion of the cold-rolled annealed plate, embedded in a resin so that a cross section parallel to the rolling direction was exposed, and the surface was mirror-polished. A tissue image (reflected electron image) of an area of 200 μm × 200 μm was taken at a ¼ part thickness of the embedded sample. Next, spot analysis was performed on all crystal grains present in the photographed region, and C and N concentrations were measured [acceleration voltage 15 kV, irradiation current 1 × 10 ⁻⁷ A, spot diameter: 0.5 μm]. In the spot analysis, the quantitative value was corrected based on a calibration curve measured in advance with a sample having a clear C and N content. When the C and N concentration measurement of each crystal grain is completed, the C concentration is 2C _C or more and / or compared with the content of C and N in the steel separately obtained by wet analysis (referred to as C _C and C _N ). Ferrite crystal grains having an N concentration of 2 _CN or more were determined as C · N concentrated grains. Subsequently, the area ratio of the C / N concentrated grains in the above-described structure image was calculated and used as the volume ratio of the C / N concentrated grains.
In each of the inventive examples, a composite structure (ferrite phase) of C / N concentrated grains and non-concentrated grains was obtained, and the structure other than the ferrite phase was less than 1% in volume ratio with respect to the entire structure. .

(2) Vickers hardness Vickers hardness evaluation was performed in accordance with JIS Z 2244. A test piece having a width of 10 mm and a length of 15 mm was cut out from the central portion of the cold-rolled annealed plate, embedded in a resin so that a cross section parallel to the rolling direction was exposed, and the surface was mirror-polished. Next, using a Vickers hardness tester, the hardness of the ¼ part thickness of this cross section was measured at a load of 1 kgf (≈9.8 N) at 10 points, and the average value was defined as the Vickers hardness of the steel.

(3) Breaking elongation From a cold-rolled annealed plate, a JIS No. 13B tensile test piece is taken so that the direction perpendicular to the rolling direction is the longitudinal direction of the test piece, and a tensile test is performed according to JIS Z 2241 to measure the breaking elongation. did. When the elongation at break is 30% or more, it passes with a very good elongation (◎), when it is 28% or more, it passes with a particularly excellent elongation (◎), and passes when it is 25% or more and less than 28% (○). The case of less than 25% was regarded as rejected (x).

(4) Ridging resistance A JIS No. 5 tensile specimen was sampled from a cold-rolled annealed plate so that the direction parallel to the rolling direction was the length of the specimen, and the surface was polished using # 600 emery paper. Thereafter, a tensile test was performed according to JIS Z2241, and a tensile strain of 20% was applied. Then, using a surface roughness meter in a direction perpendicular to the rolling direction at the polishing surface in the center of the parallel part of the test piece, the arithmetic average waviness Wa defined by JIS B 0601 (2001) is measured at a measurement length of 16 mm, Measurement was performed at a high cut filter wavelength of 0.8 mm and a low cut filter wavelength of 8 mm. When Wa is 2.0 μm or less, it is passed with excellent ridging characteristics (◎), when it is more than 2.0 μm and less than 2.5 μm, it is passed (O), and when it is more than 2.5 μm, it is rejected (×). did.

(5) Corrosion resistance A 60 × 100 mm test piece was taken from a cold-rolled annealed plate, a test piece was prepared by polishing the surface with # 600 emery paper and sealing the end face, and the salt water specified in JIS H8502 Subjected to spray cycle test. In the salt spray cycle test, salt spray (5 mass% NaCl, 35 ° C., spray 2 h) → dry (60 ° C., 4 h, relative humidity 40%) → wet (50 ° C., 2 h, relative humidity ≧ 95%) 1 cycle As a result, 8 cycles were performed.
Photograph the surface of the specimen after 8 cycles of salt spray cycle test, measure the rusting area on the specimen surface by image analysis, and calculate the rusting rate (( Rust area / total area of test piece) × 100 [%]) was calculated. A rusting rate of 25% or less was accepted (◯), and more than 25% was rejected (x).
The evaluation results of the above (1) to (5) are also shown in Table 2.

  From Table 2, it can be seen that all of the inventive examples are excellent in moldability and ridging resistance and also in corrosion resistance.

On the other hand, in Comparative Examples No. 25 and No. 26, since the C content or the N content is below the appropriate range, the volume ratio of the C / N concentrated grains is lowered and the ridging resistance is inferior. In Comparative Example No. 27, since the C content and the N content exceed the appropriate range, the volume ratio of the C / N concentrated grains exceeds the appropriate range, the elongation at break is inferior, and the corrosion resistance is also inferior.
In Comparative Example No. 28, since the Si content exceeds the appropriate range, the elongation at break is inferior, and the martensite phase is not sufficiently generated during hot-rolled sheet annealing, resulting in poor ridging resistance. Since comparative example No. 29 has Mn content exceeding an appropriate range, it is inferior to corrosion resistance. Comparative Example No. 30 is inferior in corrosion resistance because the Cr content is below the appropriate range. In Comparative Example No. 31, since the Cr content exceeds the appropriate range, the volume ratio of the C / N concentrated grains is below the appropriate range, and the ridging resistance is inferior.

In addition, Comparative Examples No. 32 and No. 36 are outside the proper range of the holding temperature and holding time of hot-rolled sheet annealing, and a sufficient amount of martensite phase is not generated by hot-rolled sheet annealing. Inferior. In No. 33 and No. 37, since the holding temperature of hot-rolled sheet annealing is below the appropriate range, the volume ratio of C / N concentrated grains in the cold-rolled annealed sheet is not sufficient, and the ridging resistance is inferior.
In Comparative Examples No. 34 and No. 38, the holding temperature for cold-rolled sheet annealing is below the appropriate range, so recrystallization is not sufficient, the hardness is high, and the elongation at break is inferior. In Comparative Examples No. 35 and No. 39, since the holding temperature for cold-rolled sheet annealing exceeds the appropriate range, a hard martensite phase is generated, the hardness is high, and the elongation at break is inferior.
From the above, it can be seen that according to the present invention, a stainless steel having excellent ridging resistance and formability and excellent corrosion resistance can be obtained.

  The ferritic stainless steel obtained by the present invention is particularly suitable for press-molded products mainly composed of overhangs and applications requiring high surface aesthetics, such as kitchen utensils and tableware.

Claims (8)

In mass%, C: 0.005 to 0.030%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.0%, P: 0.040% or less, S: 0.010 %: Cr: 15.5 to 18.0%, Ni: 0.01 to 1.0%, Al: 0.001 to 0.05% and N: 0.005 to 0.06%, The balance consists of the component composition of Fe and inevitable impurities,
Further, the structure is composed of a structure other than the ferrite phase and the ferrite phase having a total volume ratio of less than 1% with respect to the entire structure.
In the ferrite phase, the ferrite crystal grains satisfying one or both of C concentration: 2C _C or more and N concentration: 2C _N or more are 5% or more and 50% or less by volume ratio with respect to the entire structure,
Ferritic stainless steel having a Vickers hardness of 180 or less and a ridging height of 2.5 μm or less.
Here, C _C and C _N are the contents (mass%) of C and N in the steel, respectively.   The component composition is further one percent by mass selected from Cu: 0.01 to 1.0%, Mo: 0.01 to 0.5%, and Co: 0.01 to 0.5%. The ferritic stainless steel of Claim 1 containing 2 or more types.   The component composition is further in mass%, V: 0.01 to 0.25%, Ti: 0.001 to 0.10%, Nb: 0.001 to 0.10%, Ca: 0.0002 to 0 0020%, Mg: 0.0002 to 0.0050%, B: 0.0002 to 0.0050%, and REM: 0.01 to 0.10% selected from one or more The ferritic stainless steel according to claim 1 or 2. In the component composition, the C content is 0.005 to 0.030 mass%, the Si content is 0.25 mass% to less than 0.40 mass%, and the Mn content is 0.05 to 0.35 mass%. %
The volume fraction of the ferrite crystal grains is 5% or more and 30% or less,
The ferritic stainless steel according to any one of claims 1 to 3, wherein the elongation at break in a direction perpendicular to the rolling direction is 28% or more and the ridging height is 2.5 µm or less. In the component composition, the C content is 0.005 to 0.025 mass%, the Si content is 0.05 mass% or more and less than 0.25 mass%, and the Mn content is 0.60 to 0.90 mass. %, N content is 0.005 to 0.025 mass%,
The volume fraction of the ferrite crystal grains is 5% or more and 20% or less,
The ferritic stainless steel according to any one of claims 1 to 3, wherein a breaking elongation in a direction perpendicular to the rolling direction is 30% or more and a ridging height is 2.5 µm or less. A method for producing the ferritic stainless steel according to any one of claims 1 to 5,
Hot-rolling a steel slab comprising the component composition according to any one of claims 1 to 5 to form a hot-rolled sheet;
Performing the hot-rolled sheet annealing by holding the hot-rolled sheet in a temperature range of 920 ° C. or higher and 1050 ° C. or lower for 5 seconds to 15 minutes to obtain a hot-rolled annealed sheet;
Cold rolling the hot-rolled annealed plate to form a cold-rolled plate;
A method for producing ferritic stainless steel, comprising a step of performing cold-rolled sheet annealing in which the cold-rolled sheet is held in a temperature range of 800 ° C. or higher and lower than 900 ° C. for 5 seconds to 5 minutes. In the component composition, the C content is 0.005 to 0.030 mass%, the Si content is 0.25 mass% to less than 0.40 mass%, and the Mn content is 0.05 to 0.35 mass%. %
The holding temperature in the hot-rolled sheet annealing is 940 ° C. or higher and 1000 ° C. or lower,
The manufacturing method of the ferritic stainless steel of Claim 6 whose holding | maintenance temperature in the said cold rolled sheet annealing is 820 degreeC or more and less than 880 degreeC. In the component composition, the C content is 0.005 to 0.025 mass%, the Si content is 0.05 mass% or more and less than 0.25 mass%, and the Mn content is 0.60 to 0.90 mass. %, N content is 0.005 to 0.025 mass%,
The holding temperature in the hot-rolled sheet annealing is 960 ° C. or higher and 1050 ° C. or lower,
The manufacturing method of the ferritic stainless steel of Claim 6 whose holding | maintenance temperature in the said cold rolled sheet annealing is 820 degreeC or more and less than 880 degreeC.